In plants, the phloem is a vital plant organ for regulating the development of each plant organ through movement of photosynthetic products and particularly, for accumulating the photosynthetic products in plant storage organs which human beings need. The phloem differentiates into sieve elements through which photosynthetic products directly move from procambium stem cells inducing the formation of vascular bundles, companion cells known to supply ATPs, proteins, and the like to the sieve elements. The sieve elements and the companion cells make pairs to promote the movement of the photosynthetic products.
Also, the plant phloem is an organ that plays the most important role in moving products synthesized in photosynthesis to storage organs of the plant. The plant storage organs have much agricultural and economic importance as a fodder for animals. That is, when the functions of the phloem are improved, it is expected to achieve more efficient movement of photosynthetic products to the plant storage organs, thereby increasing the yield of crops.
However, very little is known about genes involved in the formation of the phloem so far, and only a few related genes have just been identified by genetic experiment methods using a plant model of Arabidopsis thaliana. Among the known genes, an APL gene is an important gene to induce the phloem formation, and thus it has been reported that the phloem is not formed when this gene is knocked out (Bonke et al., 2003, Nature 426:181-186). Also, in the case of a newly identified gene named “OCTOPUS”, when the new gene has problems with gene functions, it has been reported that the differentiation of cambium cells into differentiated cells of the phloem is delayed (Truernit et al., 2012, Development 139:1306-1315). In addition, it has been reported that the number of the phloem cells slightly increases when the signaling and biosynthesis of brassinosteroids as plant hormones are problematic in a mechanism of regulating the number of the phloem cells (Cano-Delgado et al., 2004, Development 131:3445-3455). However, there has been almost no molecular-level research on how phloem development is controlled in plants.
Therefore, a molecular-level research on novel genes involved in regulating the development and formation of the phloem in plants is needed.